The objectives of the proposed research are: 1. To employ a pulsatile nonthrombogenic left ventricular assist pump to the "bladder ventricle" type for prolonged bypass in closed chest dogs in an unheparinized system without thrombosis. The pump will be composed of a blood compatible elastomer, Avcothane-51. 2. To use left artial-aortic bypass in a synchronous 2:1 or 1:1 manner at a flow rate of 50-95% of the cardiac output in animal studies over a 5 day period. Measurements will be made of hematologic and cardiovascular function. Hematologic studies will monitor abnormalities in blood clotting and cellular morphology and changes due to excessive hemolysis and bibrinolysis. The status of the cardiovascular system will be followed by pressure and flow measurements whild bypass is in progress.